In gas turbines, during operation, heavy thermo acoustic (i.e. pressure) pulsations can occur in the combustion chamber, because of an incorrect combustion of the fuel (such as gas or oil).
These pulsations subject the hardware of the combustion device and the turbine to heavy mechanical vibrations that can result in the damage of individual parts of the combustion device or turbine.
In order to absorb such pulsations, combustion devices are usually provided with dampers, such as the Helmholtz dampers.
Helmholtz dampers consist of a resonance chamber that is connected via a damping tube to the interior of the combustion chamber (or the medium surrounding the combustion chamber).
When the volume of the chamber, the length of the tube and the area of the tube are in a defined ratio with each other, such a system is able to damp acoustic pulsations (i.e. pressure pulsations) in a certain frequency band.
Usual reheat combustion devices have one Helmholtz damper with the tube connected to the inner of the combustion chamber.
Nevertheless, as these systems only have one single Helmholtz damper for each device (therefore the damping area, corresponding to the cross section of the tube, is very small when compared with the total area of the device exposed to acoustic pulsations), their damping effect is very poor.
US2005/0229581 describes a reheat combustion device that has a mixing tube followed by a combustion chamber; the mixing tube has at its front panel an acoustic screen provided with holes and, parallel to it, an impingement plate also provided with holes.
The acoustic screen and the impingement plate define a chamber connected to the inner of the combustion chamber (via the holes of the acoustic screen) and to the outer of the combustion chamber (via the holes of the impingement plate).
During operation, air (from the compressor) passes through the holes of the impingement plate, impinges on the acoustic screen and then enters the combustion chamber; this lets the acoustic screen and the impingement plate be cooled.
Moreover, the chamber between the impingement plate and acoustic screen defines a plurality of Helmholtz dampers such that, since a plurality of dampers are associated to each reheat combustion device, the damping effect is improved.
Nevertheless, also this damping system has a plurality of drawbacks.
In fact, during operation hot gases may enter from the combustion chamber into the chamber between the impingement plate and the acoustic screen and go out again, coming back into the combustion chamber.
Usually when this occurs, the hot gases recirculate passing through two adjacent holes of the acoustic screen; this phenomenon is known as ingestion.
If ingestion occurs, the hot air flow that recirculates makes the acoustic screen and impingement plate burn in a very short time.
This could be prevented by increasing the air entering from the outside into the chamber between the impingement plate and acoustic screen through the holes of the impingement plate, but this would cause the air within the combustion chamber, that does not take part in the combustion, be increased and, consequently, the NOx emissions be increased.
A further drawback of ingestion is that of detuning of the acoustic damper.
In fact, as the temperature increases in case of hot gas ingestion, the speed of sound also increases in the damping device and, for a given geometry, the range of efficient damping is shifted off the target pulsation frequency. This makes the damper acoustically inefficient.
Moreover, as the air flow within the chamber between the impingement plate and the acoustic screen is not guided, the cooling efficiency is not optimised; this makes different parts of the combustion chamber to be cooled in different way and to operate at different temperatures.
In addition, manufacturing is very hard.